R highlighting incorrect due to bad regex

[alexr00]

Describe the bug
The existing regexes escape '.' inconsistently, e.g. we see as\.table.default. The latter '.' will match anything, so we can get weird highlighting:

To Reproduce

Can you fix this issue by yourself? (We appreciate the help)

Yes / No
This issue was originally filed by @MichaelChirico, and they made a PR to the VS Code repo: microsoft/vscode#194352